Conventional optical heads utilizing a plurality of light beams radiated from a plurality of laser diodes are described in Japanese Patent laid open Nos. 60-47238 (47238/1985), 60-113335 (113335/1985), and 60-115033 (115033/1985) which are laid open on Mar. 14, 1985, and June 19 and 21, 1985 respectively. One of the optical heads comprises three laser diodes from which three light beams each being of p-polarized wave are emitted, collimator lens through which each of the three light beams is made parallel, a polarized beam splitter for passing p-polarized wave therethrough and deflecting s-polarized wave thereby, a quarter-wave plate for conversion between linearly and circularly polarized waves, first focusing lens provided on an actuator to be controlled by a servo system so that the three light beams are focused on a record medium on which three light beam spots, for instance, a pre-beam, recording beam and reproducing beam spots are formed on a tracking line, three photodiode detecting means for detecting the three light beams each light axis having a slight slant to others after being reflected on the record medium, being passed through the quarter-wave plate and being deflected by the polarized beam splitter, and second lens for focusing the three light beams deflected by the polarized beam splitter to the corresponding photodiode means. One of the three photodiode detecting means consists of four divided regions by which one of the three light beams (for instance, corresponding to the reproducing beam spot) is received without being interrupted by a knife edge means provided between the polarized beam splitter and focusing lens. The remaining two photodiode detecting means are positioned on the knife edge means to receive the remaining two light beams (for instance, corresponding to the pre-beam and recording beam spots). The knife edge means may be replaced by a pin hole means. The construction of the optical head described above will be explained in more detail in conjunction with FIGS. 1A and 1B.
In operation, each of the three light beams of p-polarized wave emitted respectively from the three laser diodes is made parallel by the collimator lens and then passed through the polarized beam splitter. The three light beams are converted from p-polarized wave to circularly polarized wave by the quarter-wave plate and then focused on a tracking line of the record medium by the focusing lens to form a pre-beam, recording beam and reproducing beam spots thereon. The three light beams are reflected on the record medium and propagated through the first focusing lens in the opposite direction back to the quarter-wave plate. At the quarter-wave plate, the three light beams are converted from circularly polarized wave to s-polarized wave and then deflected in an approximately orthogonal direction to the light beam path between the laser diodes and record medium. One of the three light beams thus deflected is received through the second focusing lens on the four divided regions of the photodiode detecting means to produce focusing and tracking error signals which are input to the servo system. The actuator is controlled to shift the focusing lens to its positions of the light axis direction and direction orthogonal thereto thereby to correct focusing and tracking errors. The remaining two light beams are received through the second focusing lens by the two corresponding photodiode detecting means on the knife edge. Among the pre-beam, recording beam and reproducing beam spots, the pre-beam spot serves the purpose of finding whether or not a region on a tracking line to be recorded is already recorded, is not clean by the accumulation of dusts, is damaged on its surface and so on, and of detecting the address of tracking line and sector to which the region belongs. The operation of the optical head described above will be also explained later in more detail in conjunction with FIGS. 1A and 1B.
In the conventional optical head, however, it is very difficult to increase the precision in correcting focusing error due to cross talk based upon light leakage among the three light beams even if the cross talk is lowered by some specific means for the reason why the focusing error signal is produced based upon the amount of the light to be received by the two regions which are divided in the four divided regions of the photodiode means by a line orthogonal to a line on which three light beams are positioned.
For the purpose of lowering cross talk among the three light beams in correcting focusing error, a diameter of a pin hole through which one of the three light beams is passed is made smaller to restrict the amount of passing light in a case where the pin hole means is used in place of the knife edge means. As a result, the precision of correcting focusing error is lowered, although the influence of cross talk is lowered for the reason why even the amount of light including focusing error signal is restricted. In order to control the focusing lens in a stabilized manner by the servo system, a diameter of the pin hole must be as ten times as an outer diameter of the passing light beam in a practical use.
On the other hand, if the distance between the laser diodes is made larger to enlarge the interval between the light beams thereby resulting in the lowering of cross talk thereamong, the astigmatism of the focusing lens is increased. Further, if the second focusing lens is selected to have a longer focal length for the same purpose, the construction of an optical head will be larger in its size.